Butyraldehyde is produced commercially by the hydroformylation reaction of propylene, carbon monoxide and hydrogen over a suitable catalyst. The reaction effluent is comprised of the butyraldehyde product, unreacted propylene, hydrogen and carbon monoxide, and small amounts of byproducts. Similarly, n-butyl alcohol is produced commercially by the hydroformylation-hydrogenation of propylene with carbon monoxide and hydrogen in the presence of a suitable catalyst. According to one commercial process for the production of butyraldehyde, the reaction is conducted on a semi-batch basis in the mixed gas-liquid phase in a stirred reactor at a temperature of about 130.degree. C. and an absolute pressure of about 14 atmospheres. The reaction is strongly exothermic and part of the heat of reaction is removed by heat exchange and the remainder is used to evaporate the reaction products so that they can be easily removed from the reactor as a gas stream, thereby facilitating separation of the products and unreacted reactants from the catalyst residue. After exiting the reactor the gaseous product stream is cooled sufficiently to condense most of the butyraldehyde product. The noncondensed gas stream exiting the reactor, which include hydrogen, carbon monoxide, lower alkanes, including propylene and propane, and some of the butyraldehyde is conventionally recycled to the reactor. A small part of the recycle stream is purged from the system to prevent buildup of nonreactive components, such as the lower alkanes. The condensate is then stripped, preferably with steam, to remove residual volatiles (mostly propylene and propane) from the product stream. After cooling the gaseous stripper effluent to condense the steam, the volatiles stream is recycled to the reactor.
A significant disadvantage encountered in the above-described process results from the fact that industrial grade propylene usually contains small amounts, for example up to about 10% by volume, propane. Since propane is not generally affected by hydroformylation catalysts, the effluent stream usually contains propane, and the amount of propane present in the effluent may be significant, particularly when low purity propylene is used as feed. In such cases a significant volume of the volatiles must be purged to prevent buildup of propane in the system. Unfortunately, some propylene and butyraldehyde are also discharged from the system in the purge stream.
Because of the difficulty of separating propylene from propane, efficient operation of propylene recycle hydroformylation processes is hard to achieve when the propylene feed contains propane as an impurity. Continuous efforts are underway to enhance the efficiency of recycle butyraldehyde hydroformylation processes, including research investigations for improved procedures for separating propane from propylene prior to recycling the propylene to the reactor. The present invention provides such an improved procedure.